1. Field of the Invention
This invention relates to a method of recording informations on a recording medium by projecting electromagnetic radiation. More particularly, it is concerned with a recording method which is capable of performing favorable recording onto a recording medium containing therein a photoconductive substance by projecting electromagnetic radiation in a wavelength region outside that where the recording medium has its principal sensitivity.
2. Description of Prior Arts
As the method for recording information by light, there has been developed and widely used the electrophotographic method. Since this method performs the image recording by reflected light from an image original so as to be useful as an image reproduction apparatus, visible light is used in the main. In recent days, however, this electrophotographic method is also utilized in such a recording device that converts image information into electrical signals, and performs the recording with light beam controlled by the electrical signals. As an actual example of such recording device, there is such one that electrical signals are converted into light signals by means of a cathode ray tube (CRT) or a laser device, and the thus obtained light is used for the image exposure in the electrophotographic method.
In the recording device wherein the electrophotographic method and laser are combined, there is usually used a gas laser, e.g., helium-neon laser, etc. However, the laser device is extremely large in size and the image modulation is also complicated.
In contrast to this, a semiconductor laser is extremely small in size, and, moreover, does not require any external modulating device, hence the modulation can be done easily and the recording apparatus as a whole can be made small in size. However, the output wavelength of the semiconductor laser, at present, is mostly outside a wavelength region of the infrared ray, and none which is capable of emitting visible light has yet been put into practice.
Exemplifying representative photosensitive members to be used in the electrophotographic system, there are Se series, CdS series, ZnO series, and so on. These photosensitive members have, in the main, their photosensitive wavelength region in the visible light wavelength region. Therefore, in order for the photosensitive member to be sensitive to the infrared ray having a wavelength longer than the visible light wavelength region, the photosensitive member of the Se series should increase its sensitivity by forming an alloy with Te, As, and so forth. In the case of the photosensitive member of the CdS series, it is generally used by being activated with Cu or Cl. However, in order to give sufficient sensitivity with respect to the infrared ray, it is necessary to further add a trivalent metal such as In, Al, etc. It is also possible to increase the sensitivity of the photosensitive member by adding CdSe in solid-solution to CdS. In the case of the photosensitive member of the ZnO series, it is activated with a sensitized coloring substance such as Rose Bengale, bromophenol blue, Crystal Violet, Rhodamin B, and so forth. In order, however, to be sensitive to the infrared ray, it is necessary to sensitize it by adding a coloring matter such as 1,1'-diethyl-1-bromo-4,4'-quinodicarbocyanine bromide, 1,1'-diethyl-2,2'-quinotollylcarbocyanine iodide, and so on.
Thus, while it is possible to give sensitivity with respect to the infrared ray to each of these photosensitive members having their principal photosensitive wavelength region in the visible light region, the present situation is such that the sensitivity is not so high as to be satisfactory. Moreover, when the sensitivity to the infrared ray is increased by the above-described sensitizing methods, the photosensitive member tends to become generally inferior in its durability, has low resistance, and thus it becomes difficult to obtain sufficient image contrast. Also, when shortage in sensitivity of the photosensitive member is to be supplemented with a light source, the light intensity must be increased. With such increase in the light intensity, the life of the light source becomes undesirably short. The present invention has been made in view of the abovedescribed disadvantages inherent in the known types of the recording methods.